The present invention relates to a method for the manufacture of coatings with a low coefficient of friction on articles such as mechanical components and tools and to articles having a corresponding coating.
Various proposals have already been made to achieve surfaces produced by means of PVD processes which have a relatively low coefficient of friction.
The international patent application with the publication number WO97/49840, for example, discloses the production of a super-hard surface on metallic, ceramic or organic base materials.
For this purpose, a carbide intermediate layer, e.g. made of TiC, is first deposited on the base material of the article. Then a relatively thick layer, consisting essentially of pure sp2 and sp3 hybridised carbon, is deposited onto the intermediate layer, with this layer being a gradient layer, in which the proportion of the sp3 hybridised carbon increases in the direction of the free surface. This gradient layer is said to have a thickness between 0.5 and 5 xcexcm. The increasing proportion of sp3 bonds is produced by increasing the bias voltage up to approximately 300 V.
Exact data on the percentage-wise proportions of sp3 bonds are not contained in the document. It is only expressed that a predominate proportion of sp3 hybridised bonds is present at the interface to the intermediate layer in order to provide the adhesion to the intermediate layer (or to the substrate if the intermediate layer is omitted) and that the sp3 bonds predominate at the free surface. The free surface of the layer thus consists of diamond-like, and thus correspondingly hard, carbon according to the statements found there.
The international application with the publication number WO99/27893, in contrast, describes an article having a coating containing carbon which consists essentially of a sequence of layers containing carbon and metal, in which the carbonxe2x80x94carbon bond is present within each carbon layer predominately in the form of the graphitic sp2 bonds. According to the statements of this document, the intermediate layers containing carbon and metal have a period of at least 3 nm. In the specific examples, an under-layer containing metal is also used which consists of titanium or chromium and has a thickness in the range between 50 nm and 200 nm. In this proposal, the carbon layers therefore consist predominately of graphitic carbon with sp2 bonds.
It is the object of the present invention to provide a method with which a coating with a low coefficient of friction and low wear, but with relatively high hardness and also relatively high elasticity can be produced on practically any article and to provide articles with such a coating.
In order to satisfy this object, provision is made for a layer sequence of alternating carbon and carbide layers to be built up on the article by means of a PVD process, with the carbide layers being formed of a carbide of at least one element in the form of a metal and/or silicon and/or boron and the carbon being produced by sputtering a target of graphite, with the method being carried out such that the layer thickness of each carbon layer of the layer sequence is in the range between approximately 1 nm and approximately 20 nm, preferably between 2 and 4 nm, with the topmost layer of the layer sequence preferably consisting of carbon and advantageously being somewhat thicker than the other layers, for example 500 nm, with the layer thickness of the carbide layers being in the range between 1 and 3 nm, preferably at approximately 2 nm and with the carbon of the carbon layer predominately having sp3 bonds.
The content of sp3 bonds is influenced by plasma parameters such as bias voltage and plasma density. The plasma density is controlled in part by the magnetic field strength, which should be selected as relatively high in the vicinity of the parts to be coated. This can be done via the electromagnetic coil which provides the field which is responsible for the operation of the corresponding cathode as an unbalanced magnetron. The ratio of sp3 to sp2 bonds can be influenced in part by the water content in the layer system.
Unlike WO99/27893, carbon layers are produced with the invention which consist predominately of sp3 bonds. These layers are made relatively thin in each case, usually below 3 nm and thus with a thickness even less than the smallest thickness given in WO99/27893. The carbide intermediate layers are also relatively thin. Although the carbon layers consisting predominately of sp3 bonds ought to have poor adhesion to the metal carbide layers below them, if one were to judge according to WO97/49840, it was found in accordance with the invention, surprisingly, that good adhesion is achieved even with predominately sp3 bonds, but that one furthermore enjoys the advantage of the higher hardness of a carbon layer consisting of sp3 bonds, and indeedxe2x80x94also surprisinglyxe2x80x94with relatively high elasticity so that crack formation and peeling of the coating is largely reduced and the coating can also tolerate surface irregularities in the friction partnersxe2x80x94without suffering mechanical damage. The coating is also resistant to impact loads.
Thanks to the relatively thin layers and the manufacture in a usual PVD plant, in which the articles to be coated on a substrate carrier are moved alternately through the vapour fluxed from the individual targets, a gradual transition is achieved from the one layer to the next, which is also advantageous for the mechanical anchoring of the layers.
The element should be selected in particular from the group of W, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta and B and Si since it is possible with these to generate a highly effective coating with a relatively low coefficient of friction and low wear rates, together with high hardness and high elasticity, on the most varied articles. W, in particular, is suitable for the manufacture of carbide layers.
The present invention also comprises an article with the features of claim 16.
The following components and tools can be given as examples for articles to be coated:
friction-type bearings, roller bearings
various engine parts such as cam followers, camshafts, rocker arms
parts of petrol and diesel injection systems such as pistons, needles, injection nozzles and disks,
gearwheels in order to reduce pitting, erosion, fatigue cracks and adhesive wear,
parts of hydraulic systems, e.g. valve slides,
moulds, e.g. injection moulding moulds and plastic moulds.
The coating in accordance with the invention can generally be applied to all surfaces of mechanical components and articles which have low friction and which should have a wear-resistant surface coating with dry lubrication properties, e.g. CrN+carbon/metal carbide coatings can be applied to moulds (plastic moulds) in order to reduce wear and to increase anti-stick properties, which is equivalent to less friction and a saving of lubricants. The coating in accordance with the invention can also be applied to TiAlN, TiN or TiCN coated parts and tools as a lubricating coating or to drills for deep-hole boring procedures as a top coating.
The problem with the drilling of deep holes is namely the insufficient supply of cooling agent to the drill tip and the long distance to carry off the chips, which leads to an increased temperature in the flute of the drill due to the higher friction; this leads in turn to early cutting failure of the drill. In this case, the coating in accordance with the invention made of carbon/metal carbide is used as a dry lubricating agent in order to reduce cutting temperatures and to improve chip removal by reduced friction in the flute.
The total thickness of the layer sequence is in the range between approximately 0.1xcexc and 20 xcexcm, preferably between 1 xcexcm and 5 xcexcm. For some applications, it is necessary to carry out thicker coatings, i.e. coatings with a total thickness of more than 10 xcexcm, for example for the coating of connecting rods of internal combustion engines. It can be seen from these details that a layer sequence having approximately 30 to 3,000 individual layers is preferred in accordance with the invention. This plurality of layers can be manufactured particularly favourably by working with an apparatus having a treatment chamber in which the carbon and metal carbide targets required for the manufacture of the layer sequence are accommodated and by providing movement of the articles so that the articles run through the vapour flux of the individual targets alternately. This is known per se in connection with PVD processes. Not only simple rotations around one axis, but also complex rotations around two, three or more axes can be carried out in order to achieve a coating of all surfaces of the articles which is as uniform as possible or to achieve a directed coating of any desired surface.
It is particularly favourable to carry out the method as a non-reactive cathode sputtering process, in particular using an unbalanced magnetron. The apparatus provided for this purpose can be designed, for example, according to the European patent 0 439 561 or according to the European patent application 99 113 848.8. The magnetic field strength is selected to be relatively high so that the field strength in the product chamber has a value of typically 300 Gauss. The field strength can be changed during the coating, e.g. by the use of electromagnetic coils which surround the cathodic sources. The ratio of the proportions of sp3 bonds to sp2 bonds can be positively influenced by the incorporation of hydrogen in the coating.
It is also particularly favourable to carry out the non-reactive cathode sputtering process in an inert gas atmosphere, in particular in an argon atmosphere. The non-reactive atmosphere of the PVD process can moreover be supplied with hydrogen or a hydrocarbon CxHy (x and y are integers), preferably in the form of C2H2, whereby hydrogen is taken into and incorporated into the coating as this contributes to a further reduction in the coefficient of friction.
To improve the adhesion of the carbon/carbide coating in accordance with the invention on the article, a cleaning of the surface by ion etching at the start of the process is recommended, as is the deposition of a Cr layer or a Ti layer as a transition layer between the article and the carbon layer/carbide coating in accordance with the invention. This transition layer, made for example of Cr, has, for example, a thickness in the range from approximately 1 nm to approximately 1 xcexcm, in particular 0.1 xcexcm to 0.5 xcexcm, and is preferably deposited such that the proportion of Cr in the transition layer is reduced by stages or continuously in the direction of the free surface of the article.
Particularly preferred embodiments of the method in accordance with the invention and of articles having a coating in accordance with the invention can be seen from the further patent claims.